The invention relates to a transmission arrangement for interchanging information data between an electrical load, adjacent to which a transmitter is located, and a converter, adjacent to which a receiver is located, via a supply cable which is connected to a sine-wave filter, comprising series inductors and filter capacitors. In particular, the loads may be electrical drive machines, or else other three-phase or single-phase loads, such as solenoid valves, switches, etc.
For a converter, it may be highly important to know data relating to the downstream loads. Means are therefore used to interchange information data between an electrical load and an equipment which is arranged between the load and the electrical supply network and is used for automatic identification of load data, for example rating plate data relating to an electrical drive machine.
The load contains a load identification unit which has at least one such digital rating plate, which is stored in an electronically legible memory means. The load identification unit is coupled via coupling impedances to the supply lines between the load and the converter, by means of which the data from a transmitter in the load identification unit is modulated, after initialization by the converter by means of a carrier frequency that is higher than the mains frequency, onto the voltage on the supply lines, that is to say onto two phase conductors of the supply cable.
By way of example, DE 197 30 492 A1 describes the storage of information relating to the drive machine, such as type information or commissioning information, in a memory unit arranged in the drive machine. This offers the advantage that information relating to the drive machine is available in a form which can be automated by reading the memory unit.
DE 102 43 563 A1 has proposed that information be transmitted from a drive machine to a controller (converter) or the like via a supply line which is used for the electrical power supply. There is advantageously therefore no need for an additional data line. The data transmission is initialized by the upstream controller, likewise via the power supply lines, in particular with a zero-voltage state of the supply lines being used for data transmission. An electrical coil or a capacitive coupling between the cable cores is used as a coupling unit.
A similar measure is known from DE 199 11 217 A1, according to which the information from a converter is modulated at a higher frequency onto a supply line, and is transmitted to at least one further converter, central computer or similar appliance.
The arrangements operate with a constant-voltage frequency converter. Constant-voltage frequency converters are used in particular for supplying electrical power at a controlled voltage and frequency to three-phase machines. They operate with a DC voltage intermediate circuit and a controlled inverter, which is equipped with power semiconductor switches. Power FETs or IGBTs are used as power semiconductor switches, and allow very high-speed switching. Although this has the advantage of very low switching losses, it also results in problems for the insulation materials because of the high dU/dt load on the connected cables between the inverters and the three-phase machine, and may even lead to electromagnetic interference with adjacent signal cables and devices.
It is therefore known for so-called sine-wave filters to be used to cope with these problems. In the simplest embodiment, a sine-wave filter comprises a three-phase series inductor and a capacitance network between an inverter and a three-phase machine. The filter produces a sinusoidal voltage between the three conductors, but effectively also acts as a short circuit for data transmission between a three-phase machine and a converter, so that data can no longer be received at the converter.
Possible ways to cope with this problem will be:                disconnection of the capacitor on startup, by means of a switch,        provision of a damping resistance and evaluation of the voltage across this damping resistance, or        measurement of the current in the motor line or in the capacitor branch.        
In the first-mentioned case, not only must an additional switch be provided, but also appropriate control electronics for the switch. In the two last-mentioned alternatives, the measurement signals are so small that it is virtually impossible to evaluate them.